Over the years sensors have been developed which include transducers that possess a specific preferred orientation in relation to an electrical field, a magnetic field, or a mechanical force to be sensed. To maximize the response of the sensor, the transducer must be oriented in the direction of this field or force. Some examples of electrical or magnetic field sensors are position and proximity sensors such as Hall effect, magnetoresistor, capacitive, and inductive sensors and electrical current field sensors. Mechanical force sensors generally measure the flow or pressure of a liquid or gas, the mechanical stress or weight of an object, or the acceleration of an object. These sensors generally have a preferred orientation of the transducer to the electrical or magnetic field or to the physical force being sensed in order to maximize the sensitivity of the transducer.
Also, there may be other extraneous electrical or magnetic fields or mechanical forces in the system. The transducer may have to be oriented relative to these extraneous fields or forces in a specific direction to reduce the sensitivity of the transducer to them. This helps to eliminate sensing errors or noise caused by the movement of other objects or caused by the presence of other fields or forces.
These sensors also conventionally employ signal conditioning circuitry or a signal conditioner to amplify or otherwise condition the transducer signal. The signal conditioner is needed, for example, because the transducer signal is usually too low in magnitude to overcome noise or contains a large offset or other error signals that overdrive sensitive monitoring equipment. Otherwise, the transducer signal is not conducive to transmission over a distance to a remotely located sensor monitoring circuit.
Additionally, the sensors are often used in mechanical systems that have restrictions on overall size, weight, structural integrity, reliability, and cost. For these reasons, the sensor is usually made as small as possible by using transducers and signal conditioners that are electronic or that contain electrical devices manufactured on semiconductor wafers.
A first significant problem with transducers and signal conditioners which are manufactured as semiconductor devices, however, is that the electrical conductivity or other operating characteristics change significantly in response to changes in temperature. This can result in a significant change in transducer output as a function of temperature. Because most of the mechanical systems in which these sensors operate can experience rather significant changes in operating temperature, the effects of these temperature changes on the sensor output constitutes an error signal and should therefore be eliminated or reduced if possible.
The elimination of this error signal is usually a function of the signal conditioner and is usually accomplished in several related ways. The transducer and signal conditioner that are to be used together in any single sensor are usually manufactured at the same time using the same manufacturing process and are located as closely as possible in relation to each other on the same semiconductor wafer. This is done primarily to make the physical proportions of the components that comprise the transducer and the signal conditioner equal or proportional in width, length, and depth of features and to have equal relative concentrations of the various semiconductor materials used to form the components. For instance, all transistor bases, emitters, and collectors will be essentially the same relative size even if they are manufactured slightly larger or smaller than intended, and will have generally the same concentrations of materials regardless of whether they are at the intended levels of these various concentrations. The electrical conductivity of any particular electrical component in the transducer or the signal conditioner is proportional to the size of its features as well as the concentration of materials from which the component is manufactured. Any two components on the wafer located in close proximity to each other with the same dimensions and formed from the same relative concentrations of materials generally will have equal electrical conductivities if they are at the same temperature. Also, any component located near another component which has the same concentrations of materials but whose dimensions are not equal but are proportional to the other component will have an electrical conductivity that is proportional in the same degree as the dimensions if they are both at the same temperature. Because the size and composition of these components are set during manufacture, any short term changes in their electrical conductivity under identical electrical conditions are generally caused only by changes in the temperature of the component.
In this manner any specific component or collection of components on the transducer required for proper operation can be duplicated in the signal conditioner at the same size or at a specific proportional scale and with equal concentrations of materials. For example, some transducers employ four resistors in a Wheatstone bridge configuration. Any one or more of these resistors can be made with equal dimensions and with equal composition of materials on the signal conditioner. Under these conditions, the electrical conductivity of both pairs of resistors generally will be equal if their temperatures are equal. In any case, if the temperature of the transducer components is the same as the temperature of the signal conditioner components, both the transducer and signal conditioner will contain components that experience equal or proportional electrical conductivity due to the effects of temperature alone.
One of two methods are generally used in association with a signal conditioner to determine this change in electrical conductivity and then to produce a corresponding signal that cancels the effects of this change on the transducer output. First, if size allows, a complete duplicate of the transducer can be made on the signal conditioner. This duplicate transducer is then electrically, magnetically, or physically shielded from the field or force being sensed or is in some manner made unresponsive to the sensed parameter. An equal excitation or drive signal is then applied to both the components comprising the active transducer and the components comprising the duplicate passive transducer on the signal conditioner. The output of the signal conditioner passive transducer is then relative only to temperature and is then subtracted from the output of the active transducer that responds to the field or force. This is usually accomplished in a differential amplifier or a similar electronic circuit.
A second method, for example, can be used where space for the signal conditioner is more limited. A representative part of the transducer at any proportion can be duplicated on the signal conditioner. This representative part can be chosen to be a part that is unresponsive to the parameter being sensed or can be physically oriented to a position where it is not affected or otherwise shielded from the parameter being sensed. In any case, it is designed so its electrical conductivity is proportional only to changes in temperature. The change of the transducer output due to the cumulative changes of electrical conductivity of all transducer components due only to changes in temperature is determined by direct measure or by mathematical calculation during the sensor design phase. This yields a specific level of transducer output change per degree of change in temperature. This information is used to design a circuit with a specific amount of gain determined by the relationship of the change in electrical conductivity of the signal conditioner duplicate component to the change in transducer output caused by a change in temperature. This circuit monitors the change in electrical conductivity of the signal conditioner duplicate component and then amplifies this change by the amount required to yield an equivalent signal level change that is then subtracted from the transducer output as above.
Long term changes of electrical conductivity are a second significant problem in semiconductor components. This is usually caused by electromigration of the atoms of the material comprising the components from their positions as manufactured along paths of electrical current into areas that are not designed to contain them. For instance, the atoms comprising the base structure of a transistor can migrate into the areas occupied by the emitter and the collector, and vice versa. This changes both the physical size of the component as well as its concentration of the materials comprising the component. Any component in the transducer will experience these effects the same as an identical component in the signal conditioner provided the current through both components is kept equal over the life of the sensor. This is felt the same way as the short term effects of temperature as above by both components in the transducer and in the signal conditioner and is thus effectively compensated for in the same manner as short term changes in temperature.
During the manufacturing process, both the transducer and the signal conditioner are formed on a common surface on the wafer known as the planar surface. Since components are not usually formed on top of other components, this results in transducers and signal conditioners that have a large surface area relative to the depth of the devices. The area taken up by these devices is generally measured along this planar surface. The depth of all such semiconductor devices is usually fixed by design considerations and is not relative to the number of devices.
Prior art sensors generally manufacture the signal conditioner and transducer on the same wafer and interconnect the two using conductive traces defined directly on the wafer. The prior art sensors are then installed as a single monolithic chip in the sensor. Since the transducer generally should be oriented in a specific direction relative to the field being sensed, this requires that the signal conditioner be oriented also to the field in like manner.
Also, the amount of area occupied by the transducer is much smaller than the area occupied by the signal conditioner. Orientation of both a transducer and a signal conditioner along the same plane generally produces a larger cross section for the sensor than could be achieved by orienting the transducer to the field and orienting the signal conditioner separately in whatever direction needed to realize the smallest cross section. Because the signal conditioner does not require a specific orientation in relation to the field, a much smaller cross section in relation to a specific direction of measurement can be realized by changing the orientation of the transducer and signal conditioner so they are orthogonal. This can only be accomplished if the transducer and signal conditioner are physically separated and electrically connected using some means other than the conductive traces so the transducer can be oriented to the field or force separately from the signal conditioner.